Simulation of Hydraulic Fracturing Fluid Cleanup for Partially Degraded Fracturing Fluids in Unconventional Gas Reservoirs

Abstract

Abstract Unconventional reservoirs are low porosity and low permeability reservoirs (<0.1md), usually requiring enhanced stimulation techniques such as hydraulic fracturing and horizontal drilling to increase the contact between the wellbore and the producing formation for a profitable recovery. During the fracturing process, fluids are pumped into the reservoir under high pressure, to create fractures through which gas flows back to the earth’s surface during production. We expect these fracturing fluids to properly clean up during production. The inadequacy of our cleanup processes after a fracture treatment is typically due to poor degradability of our polymers, proppant crushing, clay swelling in the case of incompatible fluids and formation damage. This thesis summarizes the development of a comprehensive workflow, from characterizing an existing reservoir to simulating fluid flow and recovery performance through a fractured grid. A 2-D, three-phase IMPES simulator, incorporating a yield-power-law-rheology (Herschel-Buckley fluids), has been developed in MATLAB to characterize fluid flow through a hydraulic fractured grid and assess the influence of increasing breaker activity on yield stress and broken gel viscosity, varying polymer concentration along the fracture face, fracture conductivity, fracture length and capillary pressure on the fracturing-fluid cleanup process and cumulative fluid recovery in tight gas reservoirs. The effect of increasing capillary pressure in the formation simulated in this study resulted in a 10.4% decrease in cumulative production after 100 days of fluid recovery. Increasing the breaker concentration from 5-15 gal/Mgal on the yield stress and fluid viscosity of a 200 lb/Mgal guar fluid resulted in a 10.83% increase in cumulative gas production. Several correlations have been developed relating polymer concentration to distance along the fracture face and injection time. The rate at which the yield stress (τ_o) is increasing is found to be proportional to the square of the volume of fluid lost to the formation. For tight gas formations (k=0.05 md), fluid recovery increases with increasing shut-in time, increasing fracture conductivity and fracture length, irrespective of the yield stress of the fracturing fluid. Mechanical induced formation damage combined with hydraulic damage tends to be the most significant.